It has been known for some time that reserve batteries which employ lithium as an anode, methyl formate as the electrolyte solvent, and lithium hexafluroarsenate as the electrolyte salt, plus a cathode, are not stable and are particularly not stable at high temperatures. When a reserve cell is constructed so as to isolate the electrolyte, adverse chemical changes in the electrolyte take place which materially alter the chemical makeup of the electrolyte and cause a build-up of pressure which most often ruptures or otherwise destroys the container holding the electrolyte.
At the same time, these unstable electrolyte systems produce an extremely high energy due to the high conductivity of the lithium hexafluroarsenate in the solvent methyl formate. Many times design limitations require the high degree of energy that this chemical system can produce, particularly in airborne applications where additional weight is undesirable, and yet the undesirable instability causes frequent replacement and repair of damaged reserve cells.
One solution which has been proposed as a way of improving the stability of lithium hexafluroarsenate/methyl formate electrochemical cells is disclosed in U.S. Pat. No. 3,887,397. In this patent, a minor amount of lithium tetrafluroborate is included in the electrolyte to improve the stability. Nevertheless, long term storage at extremely high temperatures still results in an unacceptable high number of failures of reserve cells employing the highly conductive lithium hexafluroarsenate/methyl formate electrolyte.
Accordingly, it is an object of this invention to provide an improved stability system for lithium batteries which employ lithium hexafluroarsenate and methyl formate as the electrolyte system.
Other objects will appear hereinafter.